The present invention relates to a method of and an apparatus for controlling the operation of a compressor system having a plurality of compressors connected in parallel, each having the function of changing its capacity. More particularly, the invention is concerned with a method of and an apparatus for controlling the capacities of the compressors and the number of compressors taking part in the parallel running in accordance with changes in the load.
Compressed fluids such as air are used as the power source in various facilities such as a machine or chemical plant, civil engineering or construction sites, and so forth. Usually, in such a use, the composition or supply rate of the compressed fluid, i.e., the load level, changes widely. In order to fulfill the demand for a wide variation in the load level, the system for supplying the compressed fluid usually has a plurality of compressors connected in parallel and having a total capacity large enough to meet the maximum load demand, and the total discharge rate is changed in accordance with changes in the load level while maintaining a constant fluid pressure, thus economizing on the consumption of power. Basically, this control is achieved mainly in either one of the following two ways: a number control method in which the number of compressors taking part in the parallel running is controlled to meet the varying load demand, and a pressure control method in which the discharge rates, i.e., the capacities of the compressors, are controlled by controlling the operation of capacity controllers associated with respective compressor in accordance with changes in the load. The first-mentioned method is disadvantageous in that the rate of supply of the fluid is drastically changed in a non-linear manner because the control is made in a rather rough manner by changing the number of compressors taking part in the operation and also in that the supply rate cannot be changed quickly following the load variation due to various restrictions concerning the starting and stopping of the compressors. The second-mentioned method also suffers from disadvantages such as heavy wear of the capacity controllers and a resulting reduction in efficiency, as well as shortening of the life of compressors, due to partial load operation of all compressors.
In view of the above, it is preferable to combine these two types of controlling method. To this end, hitherto, it has been practiced to adopt separate loops: namely, a capacity control loop for controlling the capacities of the compressors and a compressor control loop for controlling the number of compressors taking part in the operation. In operation, the capacity control loop serves to comply with comparatively small changes in the load demand, while the compressor control loop is used when the load changes rather drastically. This combined system, however, tends to cause a hunting of the control system because of lack of communication between the two loops. Consequently, the frequency of controlling changes tends to be increased, which shortens the life of the capacity controllers and the compressors themselves.
In Japanese Patent Publication No. 30990/1982, a control system having a combination of the compressor control method and the pressure control method is proposed by inventors some of whom are also inventors of the present application. According to this control system, the compressors are put into operation in a sequence and are put out of operation in the same sequence. That is, the compressor which has worked longest of the compressors under operation is scheduled to be the one first stopped when a stopping instruction is given. When a decrease in the load is comparatively small, the capacity is reduced in a stepped manner only in the compressor which is due to be stopped first, while the other compressors are operated at full load, whereas, when the reduction of the load is large, the compressor due to be stopped first is stopped without delay. In this control system, the compressor control loop and the capacity control loop are related to each other to meet varying load demands, but the requirement for a delicate control of the supply rate is not fully met because the capacity control is made in a stepped manner in only one compressor.
Under these circumstances, there is an increasing demand for a method of controlling the operation of compressor which permits a control of the rate of supply of the fluid in a delicate manner and over a wide range, while suppressing the wear of the capacity controllers and prolonging the life of the compressors.